/Engine/Source/Runtime/Engine/Public/InstancedStaticMeshDelegates.h
This is only fired for instances added or removed via the various Add/Remove/ClearInstances functions, and not for instances updated via serialization or undo/redo.
typedef TMulticastDelegate_TwoParams< void, [UInstancedStaticMeshComponent](API\Runtime\Engine\Components\UInstancedStaticMeshComponent) *, [TArrayView](API\Runtime\Core\Containers\TArrayView)< const [FInstanceIndexUpdateData](API\Runtime\Engine\FInstancedStaticMeshDelegates\FInstanceIndexUpdateData) > > FOnInstanceIndexUpdated